Today we built up a model of the centripetal force. It was one of those “let’s trust that data, because I say so” models. The students have a lot more confidence when their data gives them a clear and straightforward relationship, so this was unfortunate.
The problem was with our apparatus. Lacking anything fancy, and also lacking the time to build Rex Rice’s excellent device, we did it with the simplest apparatus one could imagine. Here’s a schematic sketch:
There are plenty of problems with this. First is the difficulty in using it: one student holds the force gauge in one hand and also uses the straw to impart rotational momentum to the stopper. This is tricky! Second is the friction between the string and the top of the straw. Most significantly, however, is the difficulty (nay, impossibility!) of the hand in maintaining a constant stirring motion with the straw — this causes the force gauge to read a wildly-fluctuating value, and makes it very hard to ensure that the system is operating with a regular angular velocity.
My own pre-lab trials were iffy, but of course the students were much less careful with their own measurements. As a result, we got one F = kv^2 graph, one F = km graph, and one F = k/R graph. That’s just barely enough to convince that the form should be F = mV^2/R, but we were nowhere close being able to see that the slopes had physical meanings.
Next year, I’ll try to be a bit more on-the-ball about getting the device built. I’m also going to look into getting some more-accurate force gauges — these are okay, but don’t give the sort of accuracy we need. Maybe Vernier…